1. Field of the Invention
The field of the invention is programmable controllers, including those controllers which are integrated into numerical control systems as programmable interfaces.
2. Background of the Invention
In programmable controllers and interfaces a random-access read/write memory (RAM) is used to store input/output status data and a user control program, so that the I/O status can be frequently updated and the control program can be easily revised when necessary. The RAM, however, is volatile, i.e., it must be energized at all times to prevent a loss of stored data. Power is provided to the RAM by a main power supply which is connected to an a-c power source and which converts a-c power to d-c power for operation of the RAM.
Memory back-up batteries have been employed in prior programmable controllers to provide standby d-c power to the RAM when a-c power to the main power supply is interrupted. These batteries typically are limited to supplying standby power for a month and have a battery life of one to three years. Even with memory back-up batteries, data can be lost if these battery limits are reached. These batteries also require charging circuits to enable recharging during system operation from an a-c power source.